Re: [BUG] kernel BUG in ext4_do_writepages

[yebin (H)]

On 2026/3/6 13:42, Xianying Wang wrote:
> Hi,
>
> I would like to report a kernel BUG triggered by a syzkaller
> reproducer in the ext4 filesystem writeback path.

There was a period when I also noticed block allocation failures during
write-back, but after the configuration was changed, it didn't seem to
happen again.

> The issue was originally observed on Linux 6.19.0-rc8 and can also be
> reproduced on Linux 7.0-rc2. The crash occurs in the ext4 writeback

Can you identify which patch or which patchset introduced the issue?

> routine while the background writeback worker is flushing dirty pages
> to disk.
>
> During the crash, the filesystem reports that no free blocks are
> available while dirty pages and reserved blocks still exist. Under
> this condition, the writeback worker continues processing pending
> writeback operations and eventually reaches an internal consistency
> check inside the ext4 writeback routine, which triggers a kernel BUG.
>
> Based on the execution context, the issue appears to be related to the
> interaction between delayed allocation and the writeback mechanism
> when the filesystem runs out of available blocks. When the writeback
> thread attempts to flush dirty pages in this state, ext4 enters an
> unexpected internal state that causes the BUG to be triggered.
>
> This can be reproduced on:
>
> HEAD commit:
>
> 11439c4635edd669ae435eec308f4ab8a0804808
>
> report: https://pastebin.com/raw/dNFvCatE
>
> console output : https://pastebin.com/raw/LAPYKL5P
>
> kernel config : https://pastebin.com/7hk2cU0G
>
> C reproducer :https://pastebin.com/raw/v07yFCWP

Can you add these to the email as attachments?

> Let me know if you need more details or testing.
>
> Best regards,
>
> Xianying
>
>
> .